CODEWITHSUNDEEP
directx-11hlsl
Russell Trahan
Russell Trahan

Reputation: 791

HLSL Shader Optimilation with MAD(m,a,d)

I understand that the expression x = m*a+d is most efficiently written as x = mad(m,a,d) because at the assembly level only one instruction is needed rather than a multiply and add separately. My question regards optimally writing this expression: x += m*a. Should this be written as x = mad(m,a,x) or x += m*a? The difference is too subtle to profile but I'm wondering if anyone can see the difference at the assembly level. (I don't know how to view the assembly code.)

Upvotes: 0

Views: 1738

Answers (1)

Adam Miles
Adam Miles

Reputation: 3584

Compile the shader with 'fxc' and have a look at the compiled DXBC it prints out.

In this case, the compiler has no trouble determining that x += m * a; is the same as x = mad(m,a,x); and so the resulting generated code will be the same.

For both expressions the resulting bytecode is produced:

mad o0.x, v0.x, v0.y, v0.z

Even when written as x = m * a + d; the compiler is free to (and does) use a mad instruction in the DirectX bytecode. There is no performance penalty to writing it in any of the three ways you described.

Upvotes: 1

Related Questions